The present invention broadly relates to display systems which employ multiplexing to reduce the number of switching circuits required to control the electrodes in the display device. More specifically, the invention relates to techniques for preventing interactions among closely positioned electrodes from causing erroneous displays.
In the present discussion, the term "display device" shall refer to a device which converts electrical signals into visual representations, and the term "display system" shall refer to a combination including both a display device and electrical circuitry which operates the display device by applying signals thereto.
Many display devices employ an array of symbols in such shapes as dots, rectangles or line segments to display a variety of numerals, letters, arrows, bar graphs or other characters by simultaneously displaying symbols which in combination describe the shape of the character to be displayed.
Such display devices generally include an array of electrodes, one electrode for each symbol, whereby a symbol is displayed by applying a signal to the electrode corresponding to that symbol. An electrode may be said to be activated when a signal is applied so as to display a symbol, and a signal which activates an electrode shall be referred to herein as an activating signal.
The electrical circuitry which operates the display device generally includes an array of switching circuits for selectively activating the electrodes in the display device. To permit displaying various combinations of symbols, a separate switching circuit is required for each electrode so that each electrode can be controlled independently of the others.
Multiplexing is a commonly used technique for reducing the number of switching circuits required to control each electrode independently. In a multiplexed display system, the electrodes in the display device are organized in groups, and the groups are associated with a means for enabling only one selected group at a time and disabling the other groups. When a group is enabled, every electrode in that group operates normally so that activating any electrode causes its corresponding symbol to be displayed. When a group is disabled, activating an electrode in that group has no effect, and none of the symbols corresponding to the electrodes in that group can be displayed.
In a multiplexed display system, each switching circuit connects simultaneously to one electrode in every group, so that each switching circuit selectively activates either all or none of the electrodes to which it is connected. Since only one group is enabled at a time, each switching circuit can display only one symbol at a time, which is the symbol whose corresponding electrode is in the one enabled group.
To allow any combination of symbols to be apparently displayed simultaneously, the enabling means rapidly sequences through each group to enable each group for a short period at least 60 times per second. Even though only the symbols in the one enabled group can actually be displayed at any moment in time, the sequencing from one group to the next is so rapid the sequentially displayed symbols in different groups appear to be displayed simultaneously and continuously.
A common problem with multiplexed display systems is that the enabling means generally works imperfectly. In particular, if an activated electrode is in a group that is disabled but positioned adjacent to an enabled group, the activated electrode may produce a light emission or some other erroneous display, even though its group should be disabled.
One type of display system that illustrates this problem is the type whose display devices are gas discharge display tubes. In a gas discharge display, the electrodes are cathodes and the enabling means comprises a plurality of anodes, one anode being associated with each group of cathodes. A cathode is activated by applying a certain relatively negative voltage thereto, and a group of cathodes is enabled by applying a certain relatively positive voltage to that group's anode, thereby producing a current flow from that group's anode to the activated cathodes in that group. This current flow causes the gas surrounding each activated cathode to emit light which exhibits the symbol corresponding to each activated cathode.
The problem is that current sometimes can flow from the anode of an enabled group to a nearby activated cathode in an adjacent disabled group. This can produce an undesired light emission known as positive glow or streamers, as discussed in U.S. Pat. No. 3,815,120 issued to Kanda on June 4, 1974. Prior art solutions to this problem include increasing the spacing between, or erecting barriers between, adjacent groups of cathodes, which has the disadvantage of limiting the density and compactness of the display. Prior art solutions also frequently require limiting the current through each cathode to prevent streamers, which has the disadvantage of limiting the brightness of the display.